Amid what it sees as an increase in unsafe behavior by the Russian military, the US military is concerned it might have to shoot down a Russian aircraft over Syria if the Russian plane is seen as a threat to US or coalition forces fighting ISIS.
"The greatest concern is that we could shoot down a Russian aircraft because its actions are seen as a threat to our air or ground forces," said Lt. Col. Damien Pickart, the spokesman for US Air Forces Central Command.
"As coalition leaders have repeatedly said, we're here to fight ISIS -- not the Russians or anyone else," he added.
The concern comes as the US military has accused Russia and Syrian regime aircraft of violating policies that were agreed to by the United States and Russia to prevent such accidents.
"We saw anywhere from six to eight incidents daily in late November, where Russian or Syrian aircraft crossed into our airspace on the east side of the Euphrates River," Pickart said.
"Over the past week or so it has decreased slightly, but the frequency of encounters remains a concern that we address in the daily de-confliction calls," Pickart added, referring to the pre-established hotline that allows military officers from Russia and the US-led coalition to talk to each other.
A senior US defense official told CNN last month that US military aircraft flying over Syria in the last few weeks had been subjected to unsafe flying practices by Russian aircraft on several occasions. The New York Times published additional details of those unsafe encounters on Friday.
President Donald Trump and Russian President Vladimir Putin pledged to continue to uphold those de-confliction measures in their recent joint statement issued on the sidelines of the APEC conference in Da Nang, Vietnam.
The statement said Trump and Putin "agreed to maintain open military channels of communication between military professionals to help ensure the safety of both US and Russian forces and de-confliction of partnered forces engaged in the fight against ISIS."
Officials have said that part of that arrangement involves defining where Russia and US aircraft operate, with those operations historically being separated by the Euphrates River, which runs through Syria, with US-backed forces operating to the east of the river and Russian/regime forces to the west.
But in recent days the Russian Ministry of Defense has publicly said its bombers and other aircraft have increasingly operated to the east of the river, bringing them closer to US operations.
Pickart said the area where most of the concerning encounters have occurred is in the airspace over the Middle Euphrates River Valley between the Syrian town of Mayadin and the Iraqi border, where the US-backed Syrian Democratic Forces and Iraqi government troops are fighting the remnants of ISIS.
He added that this area is "where we aim to annihilate as many of them as possible," referring to the ISIS fighters.
Pickart said that while "the conversations on the de-confliction line continue and have remained professional for the most part," he added that "what our air crew encounter in the air has not always reflected what was agreed to in those discussions."
"It's been a challenge for our pilots to discern whether this is deliberate on the part of the Russian pilots, or if these are just honest mistakes," he added.
America's top military officer, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford, spoke on the phone Wednesday with his Russian counterpart, Gen. Valery Gerasimov, according to a spokesman for the Joint Staff. However, the spokesman would not offer details as to what was discussed.
"Both have agreed to keep the details of their conversations private," US Navy Cdr. John Fage said.